This invention relates to a method for forming a pattern of stripes on the viewing panel of a cathode ray tube and more particularly to a method for forming stripes smaller than the opening of a mask member affixed to the viewing panel of a cathode ray tube.
Color television receivers for providing color displays utilize cathode ray tubes having a viewing panel whereon is disposed a screen consisting of a multitude of repetitive groupings of color-emitting phosphor materials. In the most commonly encountered type of color cathode ray tube, a shadow mask member having a multitude of openings is spaced from the inner surface of the viewing panel whereon the pattern of phosphor materials is disposed. Moreover, the openings in the shadow mask member not only provide a passageway for electron beams to impinge a pattern of phosphor material when the cathode ray tube is operational but also serve as a passageway for radiant energy in fabricating the phosphor material pattern.
As mentioned, the color-emitting phosphors are usually in repetitive groupings such as the well-known triads of red, green and blue phosphor dots for example. However, it may be noted that as the color cathode ray tube art has advanced there has been a shift in emphasis and a tendency toward structures of the striped or slotted screen construction. Thus, repetitive groupings of color-emitting phosphor stripes appears to be replacing the well-known triad dot structures in popularity.
Additionally, it is well-known that closely spaced grouping of multiple colors i.e. triads of dots or stripes, have a tendency toward dilution of the desired contrast. As a result, it is and has been common to surround each of the color-emitting phosphors with a light absorbing substantially opaque material such as graphite for example. Moreover, this opaque material encompassment of the color-emitting phosphors provides improved contrast whereby the color display is enhanced.
In order to provide this opaque material encompassment of the color-emitting phosphor materials, it is known that the viewing area of the phosphor materials must be reduced in order to provide the space intermediate the viewed phosphor materials whereat the opaque material may be disposed. Since the area whereat the phosphor materials are deposited is dependent upon the openings in the mask member, one known technique for providing a phospor viewing area smaller than the mask openings is to pre-coat the mask prior to providing the phosphor viewing areas. Thereafter, the mask coating is removed whereby the mask openings are enlarged. Another similar technique is to etch or enlarge the openings of the mask after the phosphor areas have been determined. Both techniques have been found cumbersome and expensive and have not been a popular approach to the problem.
Other known techniques include under-exposure and etching of a photosensitive coating. In the under-exposure technique, radiant energy is beamed through the openings of the mask member in an amount sufficient to provide an area of polymerized material surrounded by an area of partially polymerized material. Thereupon, the partially polymerized material is washed away to leave an area of polymerized material which is smaller than the openings of the mask member. Although the technique has been and still is used with varying degrees of success it has been found that the polymerized and partially polymerized material tends to vary in size, thickness, density and adherence which is deleterious to the necessary screen uniformity. As a result, size control of the coating tends to be erratic and uniformity difficult to obtain.
Similarly, the etching technique involves exposing of a photosensitive coating through an apertured mask to provide areas of polymerized material which are larger than the openings of the mask, developing to remove the unpolymerized material and leave a pattern of polymerized materials, etching of the polymerized materials with a dilute etching solution to erode the polymerized material, and washing to remove the eroded materials. However, such etching techniques have been found somewhat difficult to adequately control due to the nonuniformity of the polymerized material. Thus, the technique has presented difficulties in present-day cathode ray tube manufacturing processes.
In one other technique for developing a screen having a pattern thereon which has viewing areas smaller than the openings of the associated mask member, a radiant energy source is movably associated with the mask member. In the process, a radiant energy source is beamed through openings in the mask to provide areas of polymerized and nonpolymerized material. The non-polymerized material is removed and an opaque coating deposited in the area. The radiant energy source is moved and the process is repeated with the area of non-polymerized material overlapping the opaque coating. The non-polymerized material is removed and opaque material placed adjacent to and overlapping the original opaque material. Thus, an area smaller than the openings of the mask is developed whereat the phosphors are deposited. Obviously, such a process is expensive, complex and requires exacting control.